The present invention relates to an injection molding apparatus for producing molded forms having thin thickness portions by injection molding.
Generally, it is known to use an ultra-high speed injection machine in order to produce molded forms having thin thickness portions by injection molding. In this case, if gas is not discharged completely from a metal mold cavity at the time of injection molding, a short shot portion (not-filled portion) may be generated in a molded form. Accordingly, a clearance called a gasvent is formed in a metal mold as a counter measure against generation of such a short shot portion.
In such a ultra-high speed injection machine, although molten resin is injected at an ultra-high speed in the case where a molded form having a thin thickness portion is produced by injection molding, molten resin is hardened before gas in the mold cavity is completely exhausted through the clearance so that a short shot is unavoidably generated particularly in a thin thickness portion of the molder form. Although the hardening of the molten resin can be delayed if the metal mold is heated, it takes a long time for the injected molten resin before the molten resin is hardened so that the molded form cannot be taken out of the metal mold immediately to reduce the production efficiency.
Accordingly, it has been impossible to produce molded forms having thin thickness portions which are thinner than a predetermined value, while keeping high producing efficiency.
It is therefore an object of the present invention to solve the above problem to thereby provide an injection molding apparatus capable of producing molded forms having no short shot portions in their thin thickness potions.